This invention relates to a method and apparatus for measuring liquid flow through a channel, and more particularly for measuring cardiac output or blood flow.
Blood flow measurement has been used for a number of years to, among other things, assess heart-valve function, determine congenital heart defects such as holes in the walls of the heart chambers, and determine the efficiency of heart muscle. In general, measurement of blood flow is an important tool for diagnosing heart problems.
Perhaps the most common method of measuring blood flow involves employment of so-called thermodilution or thermal dilution techniques in which a known quantity of coolant liquid is injected into the blood stream and the temperature of the blood-coolant mixture is measured at a point downstream from the injection site. Knowing the temperature, mass and heat capacity of the coolant, and the temperature of the blood before injection, the mass flow rate of the blood can be determined by measuring the temperature profile of the blood-coolant mixture. The drawbacks of this method are the difficulty of accurately measuring the mass and temperature of each coolant liquid injection, the requirement of a trained doctor or nurse to make the coolant liquid injections each time a measurement is taken, and the possibility of an air bubble or bacterial contamination being injected with the coolant liquid each time a measurement is taken.
A number of methods have been proposed for overcoming the drawbacks of the above-described thermodilution technique. Included among these is the proposal for using a heating coil on a catheter to heat the blood continuously while measuring the blood temperature change by a thermistor positioned downstream from the coil. This and similar methods, however, also suffer from the problem of accurately determining and controlling the amount of electrical energy delivered to the coil to heat (or cool) the blood. It also suffers from blood temperature variations due to physiological mechanisms.
The above and other methods and apparatus for measuring blood flow are described in the following prior art patents: H. H. Khalil, U.S. Pat. No. 3,359,974, H. H. Khalil, U.S. Pat. No. 4,217,910, W. W. Webster, U.S. Pat. No. 3,726,269, A. M. Richards, U.S. Pat. No. 3,075,515, A. C. M. Gieles et al, U.S. Pat. No. 3,798,967, F. W. Kuether et at, U.S. Pat. No. 3,438,253 and A. W. Richardson, U.S. Pat. No. 3,478,588.